Executive function (EF), the ability to flexibly exercise control over thought, action, and emotion, develops rapidly in childhood and predicts a range of outcomes associated with success in adulthood, such as academic achievement, income, and health. Children's EF can be fostered through a wide range of training interventions, so it is a target area in the promotion of healthy development. Language is likely one critical factor tapped in successful training interventions. For example, children's use of overt or covert self-directed speech (termed private and inner speech, respectively) appears to play a key role in the engagement of EF in an anticipatory way, in advance of needing to execute a response. Such patterns have been observed across children's planning, task-switching, and working memory. Furthermore, such patterns appear to converge with a recently discovered developmental transition in the temporal dynamics of EF. Specifically, children shift from a more reactive form of control, engaged in the moment as needed, to a more proactive form of control, engaged in anticipation of needing it. These developments in the temporal dynamics of EF and in the use of language to engage EF in an anticipatory way may support one another, but their relationship has not been tested, nor has the relationship between private speech and subsequent inner speech. In an effort to shed light on the mechanisms underlying the development of EF, the proposed project aims to make an innovative contribution by integrating previously independent lines of research with a new theoretical approach and complementary methods and paradigms in the pursuit of two mutually informative aims. Aim 1 examines relations among private speech, proactive control and inner speech using established paradigms and individual differences and longitudinal methods. Proactive control will be assessed at multiple levels of analysis (behavioral, pupillometric, neurophysiological), to provide convergent, comprehensive data on the temporal dynamics of the shift from reactive to proactive control and how it relates to private speech. Private speech and proactive control are expected to be correlated at age 6, and these factors are expected to predict the transition to the use of inner speech at 7 years of age. Aim 2 tests questions of causality by manipulating children's use of proactive control or private speech, and examining the effect on the other process. Experimentally increasing children's proactive control is expected to increase their subsequent use of private speech, and encouraging children's private speech is expected to increase their subsequent use of proactive control. The proposed research would provide not only the first link between distinct language processes supporting EF, but also the first link between these language processes and the shift from reactive to proactive control. The knowledge gained from this project will advance the study of the development of EF, and may lead to the design of larger-scale training interventions to improve EF in disadvantaged populations.